Gamera and Charybdis, the two previous AUVs designed by the Duke Robotics Club, both possessed the strength of mobility. Wishing to continue to build AUVs in this fashion, Scylla was also given this characteristic. Much like Charybdis, Scylla was constructed with three coplanar thrusters 120° apart, enabling it to move in any direction without rotating, yielding full holonomic control. Three additional thrusters were positioned at these same locations so as to provide the AUV with vertical propulsion. This year we moved away from the symmetrical disc shape, which characterized Charybdis, and instead favored a more streamlined form that would increase stability and efficiency.
The chassis provides a large amount of support for the vehicle to which all external components of the AUV are attached. The chassis will be a 28”x8”x0.5” sheet of aluminum and will run along the length of the 28” Etube. The DVL will hang below the chassis using an aluminum washer machined to prevent its movement. The hydrophone puck is attached next to the DVL with a fixture that extends the hydrophones below the other components of the vehicle and allows for 360 degrees of clearance. The extension is necessary in order to allow for effective hydrophone sensing capabilities. In order to maintain the vehicle’s stability, vertical and horizontal fins will extend from the chassis.
Electronics and Battery Housing
Our previous AUV possessed three waterproof electronics tubes, one for the batteries, another for the cameras, and a third for the electrical components. When working with this design we found that the USB signal quality of our digital cameras degraded due to lack of electromagnetic shielding. In addition to this we found that dealing with many tubes increased the time overhead associated with water testing. For these reasons we placed all three aforementioned components in one tube. The tube is constructed of clear acrylic and is sealed with an acrylic cap and an acrylic dome. The front dome is molecular bonded to the tube while the back cap is kept sealed with bolts and double O-rings, so as to allow for internal access when out of the water. The tube is 31.5” long with a 7” internal diameter. These parameters were chosen both to allow space for all of the electrical components and to maximize the hydrodynamics of the vehicle.
Six Tecnadyne 250 propeller-based thrusters are used to control the motion of the newly designed AUV. Three are arranged in the horizontal plane and allow for motion forwards, backwards, and side-to-side. The other three are arranged vertically and allow the vehicle to descend or ascend in the water as well as correct any instability in the AUV’s pitch or roll. Since each thruster has bidirectional variable power, it is possible to move in any direction by using the correct combination of thrust.
The six thrusters, which draw water through the machine, is the inspiration for the vehicle's name. In Greek mythology Scylla was once a beautiful water nymph turned into a sixheaded monster by the sorceress Circe. Scylla lived underneath a treacherous rock at one end of the Straight of Messina and devoured any ship crewmen that dared sailed near. Scylla, opposite the strait from the sea monster Charybdis, was one of the obstacles in Homer's Odyssey.
A mathematical model was developed that determines the thrust contribution from each thruster to move in a particular direction. This model ensures that the overall net thrust vector passes through Scylla's center of mass (which is roughly the center of drag), thereby preventing a rotational moment. The Tecnadyne thrusters do not have a linear force response to voltage, nor are they symmetrically powerful, having 12 lbs thrust in the forward direction and only 6 lbs in reverse. Thus, a cubic polynomial relationship was developed to model the force voltage properties of the thrusters, taking into account these nonlinearities. The computer applies this voltage with a data acquisition card’s (DAC) analog output. In the 2004 competition Charybdis’ center thruster control chip was damaged by water in a pierced cable. To prevent such leakage, all thruster end caps were made with bulkhead connectors to guarantee full water blockage. Micro connectors were used on the thrusters and the electronics tube end cap to reduce the space consumed by connectors. A backup thruster was also acquired as a spare, as our platform depends on all six thrusters being operational for total control.
Tecnadyne 250 Thrusters Spec Sheet
Connectors and Wet/Dry Interfaces
Consistent with the team's goals of industry standard construction and components, all electrical connections and wet/dry interfaces use Subconn Micro Low Profile Series underwater connectors. These 3 and 7 pin cables can be wet mated and will not short or leak. In addition, Micro Low Profile Series connectors are produced smaller than standard connectors so as to reduce the space they occupy and enable easier and faster cable connection and disconnection. Components were either purchased with Subconn cabling, as in the case of the thrusters, or Subconn connectors were potted onto the open-ended whips, as with the DVL. The acrylic end cap of the AUV’s electrical tube possesses only Subconn bulkhead connectors to ensure water never enters.
The dry acrylic tube containing all
electrical components of the vehicle creates
positive net buoyancy favoring the front of the
vehicle. This was countered by placing the
DVL 10 mm from the center of the vehicle
towards the front. However, once completed,
the AUV was found to be overly positively
buoyant. To rectify this problem two weights
were designed and installed on the vehicle to
decrease its buoyancy while preserving the
vehicle’s balance and hydrodynamics. Scylla
was made slightly positively buoyant such that
in the event of a power loss the vehicle might
float to the surface.
Three stabilizer fins are attached to the vehicle to prevent sudden changes in pitch, yaw, and roll. One extends vertically below the vehicle to control yaw and roll and two extend horizontally on either side of the AUV to control pitch. The wings are made of 20% glass-filled polycarbonate, which was chosen for its strength in structural applications.